I recently had to get a SMOG test for my 86' 22R PU. It barely failed CO, by 0.08, from the maximum allowed. Everything else was within normal limits if not below. I don't use this PU that much, it's my HomeDepot beater. The last oil change was 6 months ago. I've already replaced the air filter and the plugs and wires are about 6 months old.
Can just doing an oil change get me below the maximum CO level?